In video surveillance situations, it is oftentimes desirable to monitor a number of remote locations, such as entrances/exits of a building or stations along a production line, from a centralized monitoring location. For these situations, separate video cameras are stationed at each respective location to produce a view of the monitored location. If the view on each camera changes slowly, it is possible to use a single monitor to display on a time-shared basis the images produced by the cameras. To reduce system complexity and thereby mitigate operational expenses, the cameras operate asynchronously. Moreover, because the monitor is being switched from camera to camera, the monitor is also generally only synchronized with the camera image being displayed thereon.
With the introduction of advanced digital processing techniques, the video pictures generated by the cameras are processed digitally in order to store or resynchronize the image. Consequently, digital memories having large storage capacity and high input and output data rates are required. However, large memories with fast data rates are generally costly. Moreover, if the video source is not synchronized to the digital system output, as is the case in certain surveillance applications, then the memories must be operated in their slower random access mode rather than their faster page mode. In page mode, reading from the output of the memory may occur simultaneously with writing to the input of the memory. This occurs because as output samples are read out, input samples may overwrite the output sample locations at the same rate. Impliedly then the input and output must be synchronized. Random access mode is slower because, in addition to a read/write interval, an access interval is required to position the memory pointer to the desired location. This random mode accommodates asynchronous operation of camera and monitor since it is necessary to access different areas in memory in order to store picture element (pixel) data representative of the video input signal and to extract other pixel data for display on the monitor.
One conventional way to increase image resolution even for unsynchronized input and output video signals is to utilize a dual-port RAM circuit. With such a circuit, the picture data required to construct the video signal for the monitor is read from one port of the RAM whereas the pixel data produced by a given camera serves as the input for the other port. Such a memory has a deleterious property, inasmuch as the input and output ports are not completely independent of one another. Each must be interrupted periodically to effect a data transfer. In asynchronous applications, this transfer will cause an unwritten or unread region in one of the images and, accordingly, a distorted output image on the monitor. Conventionally, one way to overcome this effect is to provide two large, high-speed memories, with one serving to store input samples and the other to store output samples. Appropriate areas of memory may be transferred from one memory to the other during blanking intervals. However, implementing a system with two full memories can be costly and introduces an added degree of complexity.